The invention relates to fabricating a helical fiber sheet.
A particular field of application of the invention is fabricating helical fiber sheets for making annular three-dimensional fiber structures, such as reinforcing fiber structures for annular parts made of composite material, in particular brake disks.
Brake disks made of composite material, in particular of composite material comprising carbon fiber reinforcement and a carbon matrix (carbon/carbon or C/C composites) or having a carbon fiber reinforcement with a matrix that is at least in part made of ceramic (carbon/ceramic composites or C/C-ceramic composites), are well known. Fabricating them comprises making an annular fiber preform and then densifying it with a matrix, densification being obtained using a liquid method (impregnating with a resin that constitutes a precursor for the matrix, followed by pyrolyzing the resin), or by a gas method (chemical vapor infiltration).
The fiber preform is usually made by stacking fiber plies and bonding the plies to one another, typically by needling.
The annular shape of the preform may be obtained by cutting out from a block made up of fiber plies stacked on one another and bonded together, or by stacking fiber plies that are of annular shape and that have been cut out from two-dimensional fabrics (woven cloth, one-directional or multi-directional fiber sheets, felts, . . . ). This leads to considerable amounts of scrap material. The amount of scrap can be reduced by making plies that are annular by juxtaposing ring sectors cut out from a two-dimensional fabric, but that is at the price of an implementation that is more complex. Reference can be made in particular to the following documents: U.S. Pat. No. 4,790,052, U.S. Pat. No. 5,792,715, and EP 0 232 059.
Methods have been proposed enabling an annular preform to be obtained having the desired shape with practically no scrap material, such methods consisting in winding a helical fiber fabric with turns that are superposed flat, which turns are bonded to one another, in particular by needling. Such methods making use of a deformable helical sheet or of flattened helical braids are described in particular in documents U.S. Pat. No. 6,009,605 and U.S. Pat. No. 5,662,855. Reference can also be made to documents U.S. Pat. No. 6,363,593 and U.S. Pat. No. 6,367,130 which relate to an installation for making such annular fiber preforms from helical two-dimensional fabrics.
Those methods for obtaining annular fiber preforms require helical braids or woven sheets to be made by methods that are relatively expensive.
Proposals have been made in document WO 98/49382 to make a thick annular fiber preform directly from free fibers without going via a step of fabricating fiber fabrics, in particular by lapping a set of free fibers in the radial direction across a bed of disordered free fibers moved into the circumferential direction, the fibers being bonded by needling. Apart from the fact that the description is practically silent about the way in which the bed of disordered fibers is made and the way in which the radial free fibers are deposited, it can immediately be seen that it is practically impossible to obtain a fiber preform that does not present a high degree of non-uniformity in thickness and in fiber density.